1. Field of the Invention
The present invention relates generally to rotary bits for drilling subterranean formations. More specifically, the invention relates to fixed cutter, or so-called "drag," bits employing superabrasive cutters exhibiting varying cutting edge chamfer geometries at different locations or zones on the face of the bit, the variations being tailored to enhance durability of individual cutters while not curtailing rate of penetration (ROP) of the bit.
2. State of the Art
For some time, it has been known that forming a noticeable, annular chamfer on the cutting edge of the diamond table of a polycrystalline diamond compact (PDC) cutter has enhanced durability of the diamond table, reducing its tendency to spall and fracture during the initial stages of a drilling operation before a wear flat has formed on the side of the diamond table and supporting substrate contacting the formation being drilled. It is believed that such a feature offers similar benefits to superabrasive cutters other than PDCs, such as thermally stable PDCs and cubic boron nitride compacts.
U.S. Pat. No. Re 32,036 to Dennis discloses such a chamfered cutting edge, disc-shaped PDC cutter comprising a polycrystalline diamond table formed under high pressure and high temperature conditions onto a supporting substrate of tungsten carbide. For conventional PDC cutters, a conventional chamfer size (radial width) and angle would be 0.010 inch (looking at and perpendicular to the cutting face of the diamond table) oriented at a 45.degree. angle with respect to the longitudinal cutter axis, thus providing a larger radial width as measured on the chamfer surface itself. Multi-chamfered PDC cutters are also known in the art, as taught by Cooley et al. in U.S. Pat. No. 5,437,343, assigned to the assignee of the present invention. Rounded, rather than chamfered, cutting edges are also known, as disclosed in U.S. Pat. No. 5,016,718 to Tandberg.
For some period of time, the diamond tables of PDC cutters were limited in depth or thickness to about 0.030 inch or less, due to the difficulty in fabricating thicker tables of adequate quality. However, recent process improvements have provided much thicker diamond tables, in excess of 0.070 inch, up to and including 0.130 inch. Pending U.S. patent application Ser. No. 08/602,076, now U.S. Pat. No. 5,706,906, filed Feb. 15, 1996 and assigned to the assignee of the present invention, discloses and claims several configurations of a PDC cutter employing a relatively thick diamond table. Such cutters include a cutting face bearing a large chamfer or "rake land" thereon adjacent the cutting edge, which rake land may exceed 0.050 inch in width, measured radially and across the surface of the rake land itself. Other cutters exhibiting large chamfers but without diamond tables of such great depth are also known.
Recent laboratory testing as well as field tests have conclusively demonstrated that one significant parameter affecting PDC cutter durability is the cutting edge geometry. Specifically, larger leading chamfers (the first chamfer on a cutter to encounter the formation when the bit is rotated in the normal direction) provide more durable cutters. The robust character of the above-referenced "rake land" cutters corroborates these findings, but has also demonstrated a tendency to undesirably limit ROP.
The art has thus provided the design for a robust cutter structure which has yet to be fully appreciated and employed, due to demonstrated ROP limitations. Further, the art has failed thus far to appreciate any benefit in varying superabrasive, and specifically PDC, cutter chamfer geometry on a bit face so as to maximize both cutter durability and ROP of the bit on which the cutters are mounted.
The inventors herein have discovered that certain locations or zones on a bit face cut the adjacent formation face more readily than others. For example, referring to FIG. 1 of the drawings, schematically depicted is an exemplary PDC rotary drag bit 310 including a bit body 312 with a bit profile 314 extending radially outwardly from a centerline or longitudinal axis 316. In bit 310, cone 318 comprising a first region immediately surrounding the centerline cuts a generally circular, conical cutting zone 42 in the formation 40 which is much more easily cut than annular cutting zone 44, cut by a second region comprising the nose 320, flank 322 and shoulder 324 of the bit profile 314. Higher in situ stresses in the zone 44 rock adjacent nose 320, flank 322 and shoulder 324 of the second region combine with stresses induced from loading by PDC cutters 330 thereon to strengthen the rock. In contrast, the rock of zone 42 cut by center or cone first region PDC cutters 330 is largely stress relieved in comparison to that of zone 44, facilitating the shearing of cuttings therefrom. Use of a single cutter configuration in the various bit regions as in conventional bits is deficient in optimizing ROP, cutter durability, or both. Moreover, using only cutters having conventional small chamfers does not provide sufficient durability for cutters located over all regions of the bit face when drilling many formations.
Further, it is known that dynamic loading differs as to magnitude and direction with respect to cutters at different locations over the bit face, and such magnitudes and directions can be predicted with some reliability for cutter locations for a given bit profile with respect to a given rock formation type. A discussion of cutter loading and the factors affecting same, including rock strength confronting cutters at different locations over a bit face, may be found in U.S. Pat. No. 5,605,198 to Tibbitts et al., assigned to the assignee of the present invention and incorporated herein by reference.